The present invention relates to a single-chip microcomputer which possesses internally a clock-signal switching function and, more particularly, to a single-chip microcomputer capable of automatically stopping the oscillation of a main system-clock oscillation circuit after a clock signal has been switched to a sub clock-signal from a main clock-signal.
Today, one of the capabilities which is sought in a single-chip microcomputer is that of reducing power consumption.
A conventional example in which an attempt has been made to reduce power consumption is one which employs two oscillators, one being a main system-clock oscillator for a high speed operation and the other being a sub system-clock oscillator for a low speed operation.
In this conventional example, during the period in which the single-chip microcomputer is operable by the clock-signal for a low speed operation, the main system-clock oscillation circuit stops its operation and only the sub system-clock oscillation circuit which is slow in speed but in which the power consumption is small operates. Such a conventional single-chip microcomputer and a problem therein are fully explained later for assisting the understanding of the present invention.